Pulps are fibrous components that are mechanically or chemically extracted from plant tissues such as wood, and are classified into mechanical pulps which are produced by mechanical methods, chemical pulps which are produced by chemical methods, and semi-chemical pulps which are produced by a combination of these methods. In addition, chemical pulps are broadly classified into sulfite pulps and Kraft pulps depending on the chemicals used, with Kraft pulps accounting for the majority of chemical pulp produced.
Kraft pulp production processes generally comprise a digestion step, a washing step, a bleaching step and a processing step. In the digestion step, a wood chip raw material is subjected to pressurized steaming in a mixture of sodium hydroxide and sodium sulfide, thereby solubilizing non-fibrous components in the raw material, such as lignins, resins and organic acids. The steamed pulp obtained in the digestion step is sent to the washing step through a blow tank.
In the washing step, the non-fibrous components solubilized in the digestion step and residual digestion chemicals are removed from the pulp by washing/dewatering. The digestion liquor and washing effluent removed in the washing step are generally referred to as black liquor. The most common washers used in washing steps are vacuum drum washers, and it is common to use a multi-stage (counterflow rotating) washing apparatus in which 2 to 3 of these vacuum drum washers are connected in series.
Washing is ideally carried out while completely replacing the black liquor with pure water, but by doing so, a large quantity of effluent having a low concentration of black liquor is recovered and this effluent is concentrated through evaporation by means of an evaporator, thereby increasing costs. Therefore, it is necessary to reduce the quantity of pure water used as far as possible and recover an effluent having a high concentration of black liquor. For example, in the case of a three-stage washing apparatus, it is common for the black liquor recovered in the second stage drum washer to be used in the washing liquid (washer shower) in the first stage, for the black liquor recovered in the third stage drum washer to be used in the washer shower in the second stage, and for pure water to be used only in the washer shower in the third stage in order to increase washing efficiency.
Black liquor contains surface active substances such as lignins and sodium salts of resin components, and therefore causes foaming. Such foaming readily occurs during the washing step, and especially in vacuum drum washer vats and black liquor recovery tanks. As a result of the foaming, the pulp dewatering rate is reduced, the quality of the pulp per se deteriorates, and blocks of pulp sink as sheets rather than building up in the vacuum drum, which can cause a shutdown of operations.
In addition, black liquor is generally a high temperature strongly alkaline liquid having a temperature of 60 to 80° C. and a pH of 12 or higher, meaning that in cases where the black liquor overflows from the tank due to excessive foaming, safety problems can occur. The pulp (unbleached pulp) obtained in the above-mentioned washing step is then sent to the bleaching step.
In the bleaching step, the pulp is bleached by decoloring or removing colored components (mainly, lignins and pigment type substances) in the pulp, and the pulp may be imparted with physical and chemical characteristic suitable for the intended use of the pulp. A chemical such as sodium hypochlorite is used for the bleaching. In this bleaching step, the pH of the pulp is reduced, but resin components that adhered to the pulp during the strongly alkaline washing step are dissolved in the bleaching liquor and reduce the surface tension of the liquor, thereby causing vigorous foaming. This foaming causes a variety of problems, such as a reduction in the bleaching effect, a reduction in circulation speed, suspension or outflow of the bleaching raw material, and the like.
In the processing step, basic characteristics are imparted to a paper by beating, sizing, coloring (toning), and filling. Specifically, pulp fibers dispersed in water are repeatedly subjected to compression and restoration, or a sizing agent, a loading material, a coloring agent, and/or other chemicals are added to the pulp fibers.
A proposal has been made to use a silicone-based anti-foaming agent in order to suppress foaming in a pulp production process such as that described above. For example, Japanese Unexamined Patent Application Publication No. 2001-20191 discloses an anti-foaming agent that contains a polyoxyalkylene-modified silicone oil, a non-ionic surfactant other than the polyoxyalkylene-modified silicone oil, and a thickening agent comprising a water-soluble polymer in a silicone-based anti-foaming agent obtained by adding a finely powdered silica to a silicone oil.
However, because this type of non-ionic surfactant-containing silicone-based anti-foaming agent suffers from low anti-foaming agent dispersion stability in harsh environments such as high temperatures or strongly alkaline conditions, the anti-foaming agent per se forms “aggregates” (also known as “pitch” or “deposits”)) and the deposition of other organic substances in the black liquor, which are produced in the pulp production process, progresses, meaning that aggregates tend to become larger and more numerous.